Conventionally, a wireless network device such as an access point and wireless LAN adaptor is generally configured to encrypt data to be sent to a transmission channel with a common key encryption method such that a third party cannot read out the data even though the third party intercepts the data sent to the transmission channel.
In a common key encryption method, a string of codes, each of which corresponds to each of characters of a character string inputted by a user of the wireless network device, is set as the encryption key, and the data to be transmitted are encrypted using the encryption key. Meanwhile, the data are decrypted at a receiving side using the same encryption key. Therefore, even though the third party intercepts the data, the data cannot be decrypted without the same encryption key.
Meanwhile, in such a common key encryption method, in order to make it difficult for the third party to ascertain what kind of encryption key is used, it is necessary to increase the number of the character codes necessary for the encryption key so as to increase possible combinations of the string of the character codes.
However, in this case, there is a problem that the user has to input a lot of characters to the wireless network device, that is, the user is required to make much effort to configure the encryption key. As a technique to solve this problem, there is considered the following: the wireless network device is configured to perform an encrypting operation using an encryption key that consists of a lot of predetermined character codes without the encryption key being set by the user.
However, in this case, if the third party obtains such a wireless network device, the encryption key might easily be ascertained.